The StarNet Case Control Study Nicotine, Neurotransmission, and the Reward Pathway Slide 1. Nicotine, Neurotransmission, and the Reward Pathway Explain to your class that they are going to learn about the biology of nicotine addiction at the molecular level so they can understand how genes might contribute to smoking behavior. University of Washington Department of Genome Sciences

Where does nicotine go in your body? Slide 2. Where does nicotine go in your body? Ask students where nicotine goes in the body. Tell them to pair up and draw where it goes on Student Sheet A3.1.Then point out the pathway on the slide: From the mouth, down the trachea to the lungs From the lungs into the bloodstream to the heart From the heart to all parts of the body, including the brain Make sure that students recognize that nicotine goes to the brain. It goes to your lungs, and then via the bloodstream to your heart and all parts of the body, including your brain.

How does the body get rid of nicotine? Nicotine circulates throughout the body in the bloodstream. Eventually it is carried to the liver, where enzymes break it down into other compounds. These compounds are carried by the bloodstream to the kidneys and then excreted from the body in urine. Slide 3. How does the body get rid of nicotine? Explain that nicotine is carried in the bloodstream to the liver. One of the functions of the liver is detoxifying the body. Enzymes in the liver convert nicotine to compounds that dissolve in water. These compounds are carried by the bloodstream from the liver to the kidneys and are excreted as urine. Ask students to correct their drawing on Student Sheet A3.1, if necessary.

A look at the brain Different parts of the brain are involved in different functions, like vision, movement, thinking, memory. Slide 4. A look at the brain Tell students that in order to understand how nicotine interacts with the brain, they need to know more about brain structure and function. Point out that different parts of the brain have different functions, like vision, movement, memory, thinking, etc. Mention that there is a special region of the brain for “reward.” Ask students, “What effects does nicotine have in the brain?” Students may say that nicotine gives some people a “buzz” or slight “high” or helps them relax or helps them focus.

The reward pathway The reward pathway is a neural network used for reinforcing behaviors that are important for survival. Nicotine also stimulates the reward pathway. Eating makes me happy! This feels good! Slide 5. The reward pathway Tell students that the brain also has a region involved in reinforcing behaviors that are important for survival, called the reward pathway. It is a neural pathway in the brain It consists of specialized nerve calls that start in the ventral tegmental area and end in the nucleus accumbens (emotional center of the brain) and the prefrontal cortex (thinking area) It is activated by behaviors that are important for survival, such as eating Activation of the reward pathway leads to a feeling of pleasure, which reinforces these behaviors As you point out the reward pathway, don’t worry about the names of the different brain regions. To understand how this works, we need to learn about neurons and neurotransmission...

Neurons Have a cell body, dendrites, and axon Use electrical impulses and chemical signals Slide 6. Neurons Point out the different parts of a neuron (cell body, dendrites, axon, axon terminals). Explain that neurons use two kinds of signals, electric impulses (action potentials) that travel down the axon from the cell body to the axon terminals and chemical signals (neurotransmitters), which carry signals from one neuron to the next.

Model of a neuron Developed by Eric Chudler, Neuroscience for Kids Electrical impulse Neurotransmitter 2 Neurotransmitter 1 Slide 7. Model of a neuron How to use the model: 1. Have different students hold the following: neurotransmitter 1, each dendrite, cell body, electrical impulse, axon terminal. Also have students ready to catch neurotransmitter 2. 2. Have the person holding molecules of neurotransmitter 1 TOSS the plastic balls to the people who are dendrites. The "dendrite people" try to catch the plastic balls. This models the release of neurotransmitters and the attachment (binding) of neurotransmitters to receptors on dendrites. 3. When three plastic balls are caught by dendrites, the person holding the electric impulse can throw/slide the pool float down the axon. This simulates the conduction of an electrical impulse down the axon. 4. The pool float should speed down the axon toward the synaptic terminal where it will slam into the container. This should cause the release of the neurotransmitters (plastic balls) that were being held there. 5. Have the students at the end of the axon terminal try to catch the neurotransmitter 2. CAUTION: The pool float will travel very fast! Make sure that the person holding the synaptic terminal keeps his or her fingers and hands AWAY from the pool float. Axon terminal Cell body Dendrite

When neurotransmitter binds to the dendrites… Slide 8. When neurotransmitter binds to the dendrites Rapidly fire the buoy toward the axon terminal and see what happens!

Neurotransmission is… The process of transferring messages between neurons. Its purpose is to carry signals from one part of the brain to another or to other parts of the body, like muscles. Slide 9. Neurotransmission Tell students: That neurons are arranged in networks, with the axon terminals of one neuron lying close to the dendrites and cell body of other neurons (only two neurons shown in the figure) Neurotransmission goes in one direction, from the dendrites to the cell body, along the axon to the axon terminals of the first neuron and then across a space to the dendrites of the second neuron. As mentioned earlier, neurons use 1) electrical signals to send a message from the cell body to the axon terminal and 2) chemical signals, called neurotransmitters to send messages to the next neuron. Each type of neuron uses a certain kind of neurotransmitter. Some examples are: acetylcholine, serotonin, and dopamine. Synapse

The Synapse: How neurons talk to each other Neurotransmitters are chemical signals that transfer a message from one neuron to the next across the synapse. Transmitting neuron Receiving neuron Slide 10. The Synapse: How Neurons Talk to Each Other The synapse is the space between the axon terminal of one neuron and the dendrite of a neighboring neuron. Neurotransmitter is stored in vesicles in the axon terminals. When an electrical signal travels down the axon to its terminals, vesicles fuse with the cell membrane and release neurotransmitter into the synapse. Neurotransmitter binds to receptors on the dendrites of the second neuron. There are specific receptors fro each neurotransmitter. This results in an increase or decrease in the electrical signal sent down the second neurotransmitter. This is a convenient breaking point in the PowerPoint presentation if you plan to show it over two class periods.

Dopamine is the neurotransmitter in the reward pathway Stimulation of neurons in the ventral tegmental area results in release of dopamine in the nucleus accumbens (emotional center) and the prefrontal cortex (thinking). Slide11. Dopamine is the neurotransmitter in the reward pathway Remind students that the reward pathway contains neurons that have their cell bodies in the VTA and their axon terminals in the NA and prefrontal cortex Activation of neurons in the VTA results in release of the neurotransmitter dopamine in the NA and prefrontal cortex

Dopamine levels are tightly controlled through its synthesis, breakdown, and recycling back into vesicles. DDC Slide 12. Dopamine levels are tightly controlled Controlling the level of dopamine in the synapse and in neurons is critical for maintaining the responsiveness and sensitivity of neurotransmission. Dopamine concentration is controlled in several ways: Dopamine synthesis (TH and DDC) Recycling existing dopamine (DT) Dopamine breakdown (COMT and MAO) TH=Tyrosine hydroxylase DT=Dopamine transporter DOPA=dihydrophenylalanine COMT=Catechol-O-methyltransferase DDC=DOPA decarboxylase MAO=Monoamine oxidase DR=Dopamine receptor

Nicotine replaces the neurotransmitter, acetylcholine, in the reward pathway 1. Normally, acetylcholine binds to its receptor on neurons in the VTA, leading to release of dopamine in the nucleus accumbens (NA) and prefrontal cortex (PC). 2. Nicotine also binds to acetylcholine receptors in the VTA, resulting in dopamine release in the NA and PC. Explain to students that nicotine interacts with the reward pathway by binding to receptors on neurons in the VTA that normally bind the neurotransmitter, acetylcholine. Nicotine binding on these neurons results in dopamine release in the nucleus accumbens and prefrontal cortex, which may account for the relaxing effect and increase in ability to focus experienced by some smokers.

How does nicotine interact with neurotransmission and the reward pathway? This feels good! 1. Nicotine binds to acetylcholine receptors in the ventral tegmental area. 2. An electrical impulse travels down the neuron axon. 3-4. Dopamine is released at axon terminals in the nucleus accumbens and prefrontal cortex. 5. Dopamine binds to dopamine receptors on other neurons. Slide 13. How does nicotine interact with neurotransmission and the reward pathway? 1. Nicotine binds to receptors in the VTA 2. An electric impulse travels down the neuron axon 3-4. Dopamine is released at the axon terminals in the NA and prefrontal cortex (3. vesicles fuse with cell membrane; 4. dopamine is released) 5. Dopamine binds to dopamine receptors on other neurons 6. This leads to increase or decrease in electric impulse on receiving neuron, resulting in the rewarding effects often reported by smokers. 7. Dopamine is taken back up by transmitting neuron through the dopamine transporter. 6. This leads to increase or decrease in electric impulse on receiving neuron, resulting in the rewarding effects often reported by smokers. 7. Dopamine is reabsorbed through dopamine transporter.

What is the impact of nicotine interacting with the reward pathway? Addiction! Slide 14. What is the impact of nicotine interacting with the reward pathway? Give students time to respond before advancing the slide to provide the answer. Students should recognize that drugs that interact with reward pathway have the potential to be addictive. They should also recognize that the addictive properties of nicotine can lead to becoming a regular smoker and difficulty in quitting. The more people smoke, the greater their risk for smoking-related diseases.

What genes might be involved in differences in smoking behavior? On Student Sheet A3.3, circle the proteins that might account for differences in smoking behavior. Slide 15. What genes might be involved in differences in smoking behavior? Provide students with Student Sheet A3.3. Ask them to work with a partner and circle proteins that might vary among people and affect differences in smoking behavior. Remind them that variation in the proteins are caused by variation in the genes that code for the proteins. Students should be able to name: nicotine receptors, dopamine receptors, dopamine transporter, enzymes involved in synthesizing dopamine (TH and DDC), enzymes involved in breaking down dopamine (COMT and MAO), enzymes involved in breaking down nicotine. DDC

The reward pathway is involved in other addictive behaviors Cocaine slows the re-uptake of dopamine in the nucleus accumbens. Methamphetamine results in increased dopamine release as well as blocking the re-uptake of dopamine in the nucleus accumbens. Other addictive behaviors, like gambling and risk-taking involve the reward pathway. Slide 16. The reward pathway is involved in other addictive behaviors Like nicotine, other addictive drugs interact with the reward pathway and result in a pleasurable response. Cocaine slows the re-uptake of dopamine in the nucleus accumbens. Methamphetamine results in increased dopamine release as well as blocking the re-uptake of dopamine in the nucleus accumbens. Other addictive behaviors, like gambling and risk taking involve the reward pathway.

What are some benefits of identifying variants in genes associated with smoking behavior? Increasing our understanding of the addiction process Helping people understand their own risk for becoming addicted to nicotine, which may motivate them to quit smoking or never begin Helping people choose the best cessation program based on their genes Slide 17. What are some benefits of identifying variants in genes associated with smoking behavior? Discuss how knowledge about genetic variations associated with smoking behavior may be beneficial by: Increasing our understanding of the addiction process Helping people understand their own risk for becoming addicted to nicotine or other addictive drugs or behaviors Helping to choose the best cessation program for an individual based on their genes

What are some drawbacks of identifying variants in genes associated with smoking behavior? The same gene variants may also be associated with the use of other addictive drugs or addictive behaviors. By learning about genes that affect smoking behavior, people may get unwanted information about other behaviors. A gene that affects different traits is called pleiotropic. People who carry a risk variant may face discrimination getting jobs, health insurance, or life insurance. The frequency of a risk variant may be higher in a certain group, leading to stigmatization of that group. Slide 18. What are some risks of identifying variants in genes associated with smoking behavior? Use the slide to discuss possible drawbacks and define the words, pleiotropy, discrimination, and stigmatization. Pleiotropy: The ability of a variant in a single gene to affect multiple traits. For example, APOE is the gene that codes for apolipoprotein E, a protein that binds fats and cholesterol and carries them to the liver for processing. One of the three common variants for this gene, APOE4, results in increased risk for atherosclerosis and Alzheimer’s Disease. Discrimination: To give different treatment (usually negative) to a person or people based on group membership. Stigmatization: A negative label given to an individual or group based on a particular characteristic.